1. Technical Field
The present invention relates to communications systems, and more particularly, to millimeter-wave transmission lines and hybrid couplers.
2. Discussion of the Related Art
A conventional building block for use in high-frequency circuits such as radio frequency (RF) or millimeter-wave circuits is a ring hybrid or “rat-race” coupler. The ring hybrid or “rat race” coupler is a four-port device that is used as a power combiner or splitter in a variety of applications such as balanced amplification and mixing, differential clock or local-oscillator signal generation, and power combining. A conventional four-port ring hybrid coupler 110 having a microstrip transmission line is shown in FIG. 1A.
As shown in FIG. 1A, the circumference of the ring hybrid coupler 110 is 6λ/4, where λ is the wavelength that defines the center frequency of the operation of the ring hybrid coupler 110. Typically, the ring hybrid coupler 110 is designed to have an equal, for example, 3 dB power split, thus requiring the impedance of the ring hybrid coupler 110 to be two times the characteristic impedance of its ports. When a signal is incident on port 3 (e.g., 3, Σ), the signal's power is equally split between ports 2 (e.g., 2, P) and 4 (e.g., 4, N) with no power exiting port 1 (e.g., 1, Δ) or reflecting back to port 3. In this case, the signals at ports 2 and 4 are in phase; hence, port 3 is referred to as a common-mode, sum, or Σ port. When a signal is incident on port 1, its power is again equally split between ports 2 and 4 with no power exiting port 3 or reflecting back to port 1. In this case, the signals at ports 2 and 4 are now out of phase; hence, port 1 is referred to as the differential, difference, or Δ port.
Many techniques have been proposed to reduce the size of a ring hybrid coupler. These techniques include, for example, replacing the 3λ/4 section or arm by a λ/4 coupled-line section, replacing the 3λ/4 section with a lumped-element circuit, or using slow-wave transmission lines to reduce the wavelength of a propagating signal. Another technique for reducing the size of a ring hybrid coupler involves inserting a phase inverter in the 3λ/4 arm. An example of this technique is shown in FIG. 1B with a ring hybrid coupler 120 using a finite-ground coplanar waveguide (FGCPW) 160.
As shown in FIG. 1B, a phase inverter 130 is used to exchange ground 140a–d and signal 150a–b traces in a transmission line thus providing a 180-degree phase shift. This configuration reduces the length of the 3λ/4 arm to λ/4. By representing the λ/4 arm by a phase shift θ, the length of all arms of a ring hybrid can be further reduced by acknowledging that θ does not have to be 90 degrees. This will lead to a ring hybrid having a smaller circumference with a reduced bandwidth. The matching criterion for an arbitrary θ in FIG. 1B is given by equation (1):Z=Zo.[2(1−cot2θ)]0.5  (1)where Z is arm or ring impedance and Zo is port impedance.
FIG. 2A illustrates the phase inverter 130 in more detail. As shown in FIG. 2A, the phase inverter 130 includes the ground traces 140a–d, input signal trace 150a and phase-inverted signal trace 150b. In operation, the phase inverter 130 receives an input signal via the input signal trace 150a and provides an output signal via the phase-inverted signal trace 150b that is 180-degrees out of phase with the input signal.
Due to inserting a phase inverter in a ring hybrid coupler, the size of ring hybrid coupler is reduced. In addition, phase inverters can be inserted in a circuit where a 180-degree phase shift is needed, thus forcing a direct current (DC) ground onto signal lines of a ring hybrid. As a result, DC blocking capacitors are required in circuits connected to the phase inverter and in the case of the ring hybrid having a basic phase inverter all four ports of the ring hybrid are DC grounded thereby preventing the common-mode port from feeding DC signals into the ring hybrid coupler.
Although it may be advantageous to feed a DC signal into a ring hybrid coupler through its common-mode port when a common-mode biased differential amplifier employing a “rat-race” as a balun at its inputs and outputs is used, the conventional phase inverter configuration precludes this by requiring the use of blocking capacitors and feed resistors.